The use of underwater pipelines for conveyance of fluids is wide-spread. For instance, pipelines for oil or for potable water are laid on, or buried in, the sea floor of across lakes and rivers, and long submarine outfalls are used for the discharge of treated or untreated effluents offshore.
The pipes employed for such purposes should conform, so far as possible, to various requirements: resistance to external and internal corrosion; sufficient flexibility to fit the bottom morphology with minimal dredging work; high immersed weight, to give the installed pipe good stability against the action of waves and currents, and to prevent the floating of the pipeline.
Such floating may be occasioned by the accidental intrusion of air into pipelines designed for conveying water; however, floating is avoided if the overall immersed weight of the pipe is so high that there is negative buoyance even when the pipe is empty of water. For pipelines buried in mud, the immersed weight needs to exceed the weight of the mud displaced.
Furthermore, an underwater pipeline should be:
(a) sufficiently strong to prevent damage from small anchors; fishing gear, etc.,
(b) essentially free of pronounced irregularities or protrusions which may be caught by anchors, fishing gear, etc.
A uniform and reasonably smooth surface is also required when conventional underwater pipe-burying machines are to be used. Such machines are driven (on wheels) along the pipeline to be buried, so that significant protrusions from the outer wall cannot be tolerated.
Finally, the pipeline should allow the use of a reliable and relatively cheap method of laying. Among such methods, the "bottom pull" method (which is often used for concrete-coated steel pipes) is of a particular importance. In this method, the pipeline is constructed near the shoreline, single pipes being joined in "strings" and successive strings joined together ashore as the launching of the pipeline progresses. At the shoreward end, the pipeline is supported by a series of rubber wheels, which allow the forward movement to proceed with minimal resistance. The immersed pipeline moves forward (being pulled towards a barge equipped with a winch), sliding over the bed of the sea, river or lake on which it is to be laid.
During the pulling, the effective weight of the immersed pipeline is kept sufficiently small to reduce the frictional resistance on the bed, so avoiding the requirement of excessive pulling force. This is achieved, for instance, during the pulling of steel pipes by keeping them empty of water and using a concrete coating of suitable thickness. The requirements of the weighted pipeline to be laid by this method thus include:
(A) an essentially uniform outer wall--to avoid hindrance on the rubber wheels and the bed during pulling,
(B) adjustment of the immersed weight--by keeping the pipe empty of water, and
(C) sufficient strength to withstand the large pulling force that may be required.
These requirements have not hertofore been consistent with the use of plastic pipes, such pipes being weighted with spaced loads; consequently the "bottom pull" of the weighted pipeline is not feasible, nor, in practice, can the immersed weight be properly controlled. Although traction-resistant joints for plastic pipes have been developed, plastic pipelines often cannot withstand the pulling forces needed unless costly over-sizing of thickness of the pipe material is adopted.
Accordingly, despite the advantages (already listed) of plastic pipelines, their use for underwater--and particularly for submarine--laying is restricted by:
(1) limitations in tensile strength,
(2) the cost and time involved in the fitting of weights,
(3) the inadequacy of spaced weights, and
(4) the exposure to damage from anchors, fishing gear, etc. because of the limited immersed weight, absence of an adequate coating of the outer plastic surface, and the protrusions created by the spaced weights. Furthermore, these weights preclude the use of pipe-burying machines.